Computer-based automation has promoted the installation of large quantities of sophisticated electronics in environments of less than optimal quality. For example, modern telephone central offices and switching offices are relatively large buildings filled with racks of electronic circuits mostly assembled on printed circuit boards. An increasing fraction of the local telephone switching equipment is being located in remote sites, such as buried vaults. In both cases, the need for reliable operation of the electronics has dictated that the environment be relatively benign. Hence, not only are the temperature and humidity controlled, but the air circulation is controlled so as to minimize the incursion of dust from the outside while also exhausting internally generated particles and vapors.
Nonetheless, dust remains a problem for electronic circuitry, especially when its useful lifetime is measured in decades. In this discussion, dust will be considered to be any solid airborne particle regardless of source. A principal failure mechanism in printed circuits is an unacceptable increase in leakage current caused by dust bridging neighboring electrical leads which should be electrically isolated. Either the dust itself electrically conducts, or it absorbs water vapor after its deposition, resulting in electrical conduction. In either case, the accumulated dust increases leakage current. The dust may accumulate over long periods of routine operation or may be deposited by a specific disaster, which may range from a fire to a failure of the air filtration system. Usually the dust can be cleaned from the printed circuit boards, but any cleaning is labor intensive and thus partially defeats the automation effort.
Gourdine et al. have disclosed in U.S. Pat. No. 3,718,029 an automated apparatus to measure the amount of dust, but this apparatus does not directly measure the dust's conductivity. Hoenig has disclosed in U.S. Pat. No. 4,117,715 an apparatus for determining the polarity of charge carried by dust but does not determine the conductivity. The Institute for Interconnecting and Packaging Electronic Circuits has promulgated standard IPC-TM-650, which defines multi-purpose test board IPC-B-25. The board consists of an interdigitated electrode pattern printed on the surface of an insulating board. This test board is commonly used in the electronic manufacturing industry to ensure that the contaminant level remain at or below a prescribed level.